1. Field of the Invention
The present invention relates to a pattern formation method employed in manufacture of semiconductor devices, or the like, and specifically to a pattern formation method based on a double patterning technique.
2. Description of the Prior Art
Large-scale integration and downsizing of a semiconductor device has been demanding acceleration of development in lithography techniques. Presently, pattern formation is realized by means of photolithography using a mercury lamp, KrF excimer laser, ArF excimer laser, or the like, as a light source. The use of F2 laser with shorter wavelengths was once studied, but the study and development have been stopped as of now due to difficulties in exposure systems and resist materials.
Under such circumstances, a patterning method called “double patterning” has been proposed recently for improvements in miniaturization with conventional exposure wavelengths. This method realizes a desired mask pattern by two separate photomasks for exposure, thereby achieving an improved pattern contrast.
The resolution of lithography is defined by k1·λ/NA (k1: process constant, λ: exposure wavelength, NA: numerical aperture of exposure system). The double patterning can greatly improve the resolution even with the same exposure wavelength because improvements in the pattern contrast greatly decrease the value of k1.
Hereinafter, a pattern formation method based on the conventional double patterning is described with reference to FIG. 8A to FIG. 10B.
Referring to FIG. 8A, a hardmask (e.g., silicon nitride film) 202 is first formed on a semiconductor substrate 201 so as to have a thickness of about 0.12 μm.
Then, referring to FIG. 8B, a first ArF resist film 203 is formed on the hardmask 202 so as to have a thickness of about 0.15 μm. Thereafter, the first exposure is carried out via a first photomask 204 with light 205 from an ArF excimer laser having NA of 0.85. After the first exposure, the first ArF resist film 203 is heated by a hotplate to about 105° C. for 60 seconds.
Then, referring to FIG. 8C, a first resist pattern 203a is formed by developing the first ArF resist film 203 with 2.38 wt % tetramethylammonium hydroxide developer solution.
Then, referring to FIG. 8D, a first hardmask pattern 202a is formed by etching the hardmask 202 with a fluoric gas, or the like, using the first resist pattern 203a as a mask.
Then, referring to FIG. 9A, the first resist pattern 203a is removed by ashing with oxygen plasma. Thereafter, referring to FIG. 9B, a second ArF resist film 206 is formed on the first hardmask pattern 202a so as to have a thickness of about 0.15 μm.
Then, referring to FIG. 9C, the second exposure is carried out via a second photomask 207 with light 205 from an ArF excimer laser having NA of 0.85. After the second exposure, the second ArF resist film 206 is heated by the hotplate to about 105° C. for 60 seconds.
Then, referring to FIG. 9D, a second resist pattern 206a is formed by developing the second ArF resist film 206 with 2.38 wt % tetramethylammonium hydroxide developer solution.
Then, referring to FIG. 10A, the hardmask 202 is etched with a fluoric gas, or the like, using the second resist pattern 206a as a mask. Then, referring to FIG. 10B, the second resist pattern 206a is removed by ashing with oxygen plasma such that a second hardmask pattern 202b is formed.
With such two-step resist exposure and hardmask etching, the finer second hardmask pattern 202b is obtained. For example, the semiconductor substrate 201 (or a film to be etched (not shown) overlying the semiconductor substrate 201) is dry-etched using the second hardmask pattern 202b formed by double patterning as shown in FIG. 11, whereby the semiconductor substrate 201 (or film to be etched) is microprocessed.
In the second application of the material for the second ArF resist film 206 on the first hardmask pattern 202a, large irregularities over the surface of the underlying first hardmask pattern 202a could deteriorate the application characteristics. In such a case, the resolution of the second resist exposure would deteriorate, so that even the double patterning could not achieve a sufficient resolution.
A solution to such a problem disclosed in M. Maenhoudt et al., “Double Patterning scheme for sub-0.25 k1 single damascene structures at NA=0.75, λ=193 nm”, Proc. SPIE, vol. 5754, 1508 (2005) is to planarize the first hardmask pattern 202a with BARC (Bottom Anti-Reflection Coating).
Specifically, referring to FIG. 12, BARC 208 is applied over the first hardmask pattern 202a to form a flat surface, on which the second ArF resist film 206 is formed. In this way, the application characteristics are improved. This technique can prevent deterioration in resolution of the second resist exposure.